About defect phenomena in ZnO nanocrystals.

ZnO nanocrystals are receiving renewed attraction due to their multifunctional properties. Selective enhancement and tuning of their optical and electrical properties are essential for achieving novel devices with accurate sensing and conducting capabilities. The nature and type of intrinsic defects that occur in ZnO influence these properties. In this work, we investigate the intrinsic defect structure of ZnO via electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy and correlate the results with existing computational works. Mainly, the defects are analysed by taking the microscopic defect structure of the lattice into account. The results manifest the core-shell model of the defect structure in ZnO. By default, specifically for nanocrystals, oxygen vacancies localise on the surface, while zinc vacancies localise in the core. The investigations in this report demonstrate that the concentration of the intrinsic defects and their position can be tuned merely by changing the size of the nanocrystal. Additionally, the UV, green, orange and red emissions can be tuned by nanocrystal's size and post-annealing treatments.

Superbat: battery-like supercapacitor utilized by graphene foam and zinc oxide (ZnO) electrodes induced by structural defects.

The current work presents a hybrid type of energy storage device composed of both graphene foam and zinc oxide electrodes, which exhibits both the electrochemical performance of a supercapacitor with a relatively higher power density, and a battery with a relatively higher energy density as compared to each individual component as single devices. Te hybrid's improved performance was correlated to the defective structure of the electrodes. To enhance the electrochemical performance of supercapacitors, it is necessary to have a well-defined mass, shape, and surface area of electrode materials. Here, we present an original design of a mounting device that enabled precisely determining all the critical parameters of electrode materials for a particular mass and surface area. With the aid of our original setup, we produced a supercapacitor device that could also act as a battery due to its high energy density values, hence we named it as superbat. In this work, 3D graphene foam was used as the first electrode due to its large surface, while for the second electrode, ZnO nanocrystals were used due its defective structure. Paramagnetic resonance Raman and impedance spectroscopy were performed in order to understand the origin of the performance of the hybrid capacitor in more depth. In particular, we obtained a high specific capacitance value (C = 448 F g-1), which was exceptionally related not only to the quality of the synthesis but also the choice of electrode and electrolyte materials. Moreover, each component used in the construction of the hybrid supercapacitor also played a key role in to achieving high capacitance value. The results demonstrated the remarkable performance and stability of the superbat.

Synergetic effects of Fe3+ doped spinel Li4Ti5O12 nanoparticles on reduced graphene oxide for high surface electrode hybrid supercapacitors.

In this work, reduced graphene oxide (rGO) based electrode materials were developed to achieve a hybrid supercapacitor (SC) function. Therefore, several synthesis methods were developed to prepare a cost effective and environmentally friendly rGO. Additionally, to maintain the high surface area, spinel lithium titanate (sLTO) nanoparticles (NPs) were synthesized and deposited on the rGO surface to inhibit the restacking of the rGO layers on graphite. Furthermore, the adequate Fe-doping of sLTO increased the ionic conductivity and the intercalation capacity, which is necessary for a SC performance. The sLTO/rGO-composites were electrochemically analysed by chronopotentiometry and electrochemical impedance spectroscopy (EIS) to determine the stability during charge/discharge cycling and the capacity, respectively. To overcome the drawback of LTO's low conductivity values, its value has been drastically increased by Fe-doping. The results demonstrated the remarkable cycling performance of the Fe:LTO/rGO composite as well as a higher capacity compared to LTO/rGO and pure rGO-electrodes. The thermal stability, degradation and weight loss of the sLTO/rGO in the temperature range between 20 °C and 800 °C were investigated by thermogravimetry (TG)/DTA. As a conclusion, it can be stated that, increasing the ionic conductivity by Fe-doping drastically increases the hybrid capacity of the SC electrodes.

High-Capacitance Hybrid Supercapacitor Based on Multi-Colored Fluorescent Carbon-Dots.

Multi-colored, water soluble fluorescent carbon nanodots (C-Dots) with quantum yield changing from 4.6 to 18.3% were synthesized in multi-gram using dated cola beverage through a simple thermal synthesis method and implemented as conductive and ion donating supercapacitor component. Various properties of C-Dots, including size, crystal structure, morphology and surface properties along with their Raman and electron paramagnetic resonance spectra were analyzed and compared by means of their fluorescence and electronic properties. α-Manganese Oxide-Polypyrrole (PPy) nanorods decorated with C-Dots were further conducted as anode materials in a supercapacitor. Reduced graphene oxide was used as cathode along with the dicationic bis-imidazolium based ionic liquid in order to enhance the charge transfer and wetting capacity of electrode surfaces. For this purpose, we used octyl-bis(3-methylimidazolium)diiodide (C8H16BImI) synthesized by N-alkylation reaction as liquid ionic membrane electrolyte. Paramagnetic resonance and impedance spectroscopy have been undertaken in order to understand the origin of the performance of hybrid capacitor in more depth. In particular, we obtained high capacitance value (C = 17.3 µF/cm2) which is exceptionally related not only the quality of synthesis but also the choice of electrode and electrolyte materials. Moreover, each component used in the construction of the hybrid supercapacitor is also played a key role to achieve high capacitance value.

Charge transfer and surface defect healing within ZnO nanoparticle decorated graphene hybrid materials.

To harness the unique properties of graphene and ZnO nanoparticles (NPs) for novel applications, the development of graphene-ZnO nanoparticle hybrid materials has attracted great attention and is the subject of ongoing research. For this contribution, graphene-oxide-ZnO (GO-ZnO) and thiol-functionalized reduced graphene oxide-ZnO (TrGO-ZnO) nanohybrid materials were prepared by novel self-assembly processes. Based on electron paramagnetic resonance (EPR) and photoluminescence (PL) investigations on bare ZnO NPs, GO-ZnO and TrGO-ZnO hybrid materials, we found that several physical phenomena were occurring when ZnO NPs were hybridized with GO and TrGO. The electrons trapped in Zn vacancy defects (VZn(-)) within the core of ZnO NPs vanished by transfer to GO and TrGO in the hybrid materials, thus leading to the disappearance of the core signals in the EPR spectra of ZnO NPs. The thiol groups of TrGO and sulfur can effectively "heal" the oxygen vacancy (VO(+)) related surface defects of ZnO NPs while oxygen-containing functionalities have low healing ability at a synthesis temperature of 100 °C. Photoexcited electron transfer from the conduction band of ZnO NPs to graphene leads to photoluminescence (PL) quenching of near band gap emission (NBE) of both GO-ZnO and TrGO-ZnO. Simultaneously, electron transfer from graphene to defect states of ZnO NPs is the origin of enhanced green defect emission from GO-ZnO. This observation is consistent with the energy level diagram model of hybrid materials.

Controlling the exciton energy of zinc oxide (ZnO) quantum dots by changing the confinement conditions.

ZnO nanoparticles were synthesized by solid state and hydrolysis methods based on the conventional precipitation. In situ growth of ZnO nanoparticles were monitored by photoluminescence spectroscopy (PL). By the help of electron paramagnetic resonance (EPR) technique, detailed analysis of intrinsic defect structure of ZnO was given with respect to mean particle size. In nanoscale concentration of surface defects enormously increased and core defects reduced. In addition, blue-shift was observed in PL spectra at near-band-edge UV region due to e-h recombination. Calculation of band gap energies by the aid of Brus equation revealed consistent results with the experimental observations.